1. Field of the Invention
The invention relates to a window blind, more particularly to a window blind having a spring motor for concealed pull cords thereof.
2. Description of the Related Art
In U.S. Pat. No. 6,289,965, there is disclosed a conventional window blind that comprises a head rail, a bottom rail, and an expandable window covering therebetween. A pair of pull cords interconnect the head rail, the bottom rail, and the expandable window covering. A spring motor includes a frame, a drive drum, an idler gear, a take-up drum, a pair of cord spools, and a coil spring. The frame is mounted on the head rail. The drive drum is mounted rotatably on the frame and is provided with a drive gear. The idler gear is mounted rotatably on the frame and meshes with the drive gear. The take-up drum is mounted rotatably on and is concentric with the idler gear. The idler gear rotates independently of the take-up drum. Each of the cord spools is mounted rotatably on one end of the frame adjacent to a respective one of the idler gear and the drive drum, is provided with a driven gear that meshes with the respective one of the idler gear and the drive drum, and is connected to a respective one of the pull cords. The coil spring is wound on the take-up drum, has opposite ends connected to the take-up drum and the drive drum, and provides a biasing force for biasing the take-up drum to rotate in a direction for winding the pull cords on the cord spools.
The aforementioned conventional window blind achieves the purpose of concealing the pull cords with the deployment of the spring motor, and the bottom rail does not slant while being raised or lowered. However, the drive and take-up drums rotate at different speeds. In addition to this, the idler gear and the take-up drum rotate independently of one another. Further, the drive drum, the idler gear and the cord spools rotate at the same speed. This rotational speed relationship among the drive and take-up drums, the idler gear, and the cord spools results in a complicated construction for the spring motor of the conventional window blind:
Therefore, the object of the present invention is to provide a window blind that has a relatively simple spring motor for concealed pull cords thereof.
According to the present invention, a window blind comprises a head rail, a bottom rail, and an expandable window covering between the head rail and the bottom rail. A pair of pull cords interconnect the head rail, the bottom rail and the expandable window covering. A spring motor includes a frame, a drive drum, a pair of cord spools, a spiral spring, and a friction imposing mechanism. The frame is mounted on one of the head and bottom rails. The drive drum is mounted rotatably on the frame and is provided with a drive gear. Each of the cord spools is mounted rotatably on the frame, is disposed on a respective one of opposite sides of the drive drum, is provided with a driven gear that meshes with the drive gear, and is connected to a respective one of the pull cords. The spiral spring is wound on the drive drum, has opposite ends connected respectively to the drive drum and the frame, provides a biasing force for biasing the drive drum to rotate in a direction for winding the pull cords on the cord spools, and deforms from an initial state to an extent corresponding to vertical distance of the bottom rail from the head rail. The friction imposing mechanism is mounted on the frame and is operable so as to provide a friction force that acts on one of the drive drum and the pull cords. The biasing force of the spiral spring and the friction force attributed to the friction imposing mechanism cooperate to support the weight of the bottom rail and the weight of the expandable window covering that acts on the bottom rail so as to retain the bottom rail at a desired vertical distance relative to the head rail.